The flow behaviour of slurries within cyclones is quite complex. This has led designers to rely on empirical equations for predicting performance of cyclones. These empirical relationships are derived from an analysis of experimental data and include the effect of operational and geometric variables. Different sets of experimental data lead to different equations for the same basic parameters. However, these empirical models suffer from the inherent deficiency of any empirical model. That is, the model can only be used within the limits of the experimental data from which the model parameters were determined. In view of this shortcoming, mathematical models based on fluid mechanics are highly desirable.
The numerical technique “Computational Fluid Dynamics (CFD)” herein means the numerical treatment of the Navier-Stokes equations over a structured/unstructured 3D grid within a cyclone body. Turbulence modeling is achieved by both RSM (Reynolds Stress Model) and LES (Large Eddy Simulations) in order to capture the high swirling flow patterns seen in dense medium cyclones. CFD provides a means of predicting velocity profiles under a wide range of development and operating condition. The numerical treatment of the Navier-Stokes equations, the basic of any CFD technique, crept into the analysis of cyclone behaviour in the early 1980s. This resulted from the rapid Improvement in computers at that time and a better understanding of the numerical treatment of turbulence.
The expression “fine coal dense medium separation process” herein means separation of a fine coal material into dense and less high gravity fraction at a predetermined cut point. In the process the particulate material is carried in a dense liquid medium which typically comprises a mixture of water and particles of dense material such as ultra fine magnetite.
At present, the dense medium cyclone (DMC) is one of the best pieces of processing equipment for washing coal of −20+0.5 mm size. Difficult washing characteristics associated with, for example, many Indian coals are generally due to the present of a high proportion of near-gravity material (NGM). This makes the DFC an obvious choice for most Indian washers. In order to produce low ash clean coal from run-of-mine (ROM) coal, it is necessary to crush the ROM to fine sizes to liberate ash & coal. One of the more efficient methods of beneficiating the generated intermediate size fraction (−2+0.25 mm) fines is dense medium operation is small diameter cyclones.
A known prior art dense medium cyclone is illustrated in FIG. 1. This conventional dense medium cyclone 10 includes a cylindrical inlet chamber 11 into which a mixture of medium and raw coal enters tangentially through an inlet 12, thus forming a strong vertical flow. The refuse or high ash particles move along the wall 13 of the cyclone due to the centrifugal force, where the velocity is the least and is discharged through the underflow orifice 14 or spigot. The lighter washer coal moves towards the longitudinal axis 15 of the cyclone due to the drag force where a high velocity zone exists and passes through an overflow orifice or vortex finder 16, also sometimes termed an overflow chamber. With cyclone of this type, entrainment of fine or slower settling particles occurs in the void spaces between the coarser, or faster settling, particles discharged as the underflow. Turbulence fluctuation inside the cyclone are also expected to be significant due to the collision of the inlet stream with the rotating stream. Due to inadequate standard inlet and body section design, these cyclones are associated with large short-circuit flow and short residence time of the internal upward flow. The through put and performance is limited by the flow through the axial outlet. For many applications therefore, cyclones of this type are operated at a reduced feed rate, in order to obtain the required cut between the low gravity fraction and high gravity fraction.
There us need to develop a new design for a DMC for the recovery of clean coal ash (<8%) from high NGM coals, such as those seen in India. It would be particularly advantageous if a dense medium cyclone having increased efficiency could be devised that is able to produce a product with reduced ash content. It would also be advantageous if a dense medium cyclone could be devised having an increased ability to separate out fine particles efficiently.
A new improved development of DMC for efficient coal separation emphasizing a fines fraction of −2+0.25 mm has been development using a comprehensive CFD model. In particular, a CFD model of the DMC which is capable of predicting the performance of the cyclone has been developed, using Fluent, by coupling component models for the air-core, the magnetite medium Lagrangian particle tracking for particles ranging in size from 0.25 to 2 mm. This has resulted in the invention described below.